In some mechanical systems, rotating shafts are required to transmit high levels of torque at low rotational speeds and yet must still be capable of rotating at high speeds with reduced torque. An example of such a requirement is in high speed electric motors utilized in automotive coolant systems in which shaft seals are used to isolate coolant liquid from shaft support bearings while retaining lubricant within the bearings. In this situation, if coolant liquid is able to mix with lubricant, the lubricant is degraded, which can eventually result in bearing failure.
In a typical shaft, housing, bearing and seal arrangement, the seal is mounted directly on the housing with the seal-to-shaft contact point, or line, on the surface of the rotating shaft. Maximum shaft speeds for effective seal performance depend on conditions such as shaft surface finish, temperature, eccentricity, seal type, lubricants or other fluids present as well as a number of other parameters. With respect to these conditions, surface speed at the contact point between the seal and shaft is more meaningful to the function of the seal than the rotational speed of the shaft. Surface speed is directly proportional to shaft diameter.
The minimum diameter of drive shafts used in such systems is primarily a function of the maximum torque to be transmitted regardless of the speed of the shaft, If a shaft has a high speed mode as part of its operating cycle, sealing considerations become a factor as diameters are increased to accommodate higher torque.
The maximum contact point speed for commonly used high speed lip seal materials and designs is specified by Federal Mogul for its lip seals to be 3,000 feet per minute. Exemplary of a set of conditions that would result in a contact speed of 3,000 feet per minute is a one-inch diameter shaft rotating at 11,500 rpm. A higher rotational speed, or a larger diameter shaft, results in a seal-to-shaft contact speed higher than 3000 FPM and could result in less than satisfactory seal performance and durability or seal failure.